I've seen this done in various youtube videos using the console. But as I always think of this while drinking beer with buddies this method always ends in failure (For obvious reasons).I really hope they patch it in now with the "Big Picture" thing and all

Seriously-- it's possible, it's cool, and proves that YES PCs are capable of handling on-screen multiplayer. Valve PLEASE make this a thing. Patch the game and you'll be adding to the game's lifetime. More people would play for longer! I don't want to have to put up with clunky analog controls on a console to be able to play in the same place as friends! Dang I wish they would add this already! It'd be a dream come true!